Relicense to dual MIT OR Apache-2.0

This licensing scheme gives maximum flexibility for downstream users. Apache 2.0 includes an explicit patent grant and retaliation clause, which protects users from patent litigation. With dual MIT and Apache-2.0 licensing, the user may choose any of these two at their discretion.

This is a well established standard in the rust-bitcoin and Rust ecosystems:

- [getfloresta/Floresta](https://github.com/getfloresta/Floresta)
- [mit-dci/rustreexo](https://github.com/mit-dci/rustreexo)
- [bitcoindevkit/bdk](https://github.com/bitcoindevkit/bdk)
- [lightningdevkit/rust-lightning](https://github.com/lightningdevkit/rust-lightning)
- [rust-lang/rust](https://github.com/rust-lang/rust)
- [rust-lang/cargo](https://github.com/rust-lang/cargo)

In order to relicense we would need explicit approval from all contributors which have made meaningful contributions.

If you're on the below list and agree with this change please add a comment with

> I hereby re-license my previous contributions to `bdk-tx` under [Apache 2.0](https://choosealicense.com/licenses/apache-2.0/) in addition to the existing [MIT](https://choosealicense.com/licenses/mit/) license.

- [x] @noahjoeris
- [ ] @evanlinjin
- [x] @aagbotemi
- [x] @ValuedMammal
- [x] @chaitika
- [x] @nymius